Mismatch in estimated charges data from aws cloudwatch api and aws console

I was trying to monitor my AWS account using cloud watch api. When i tried to extract estimated charges data as given below.

cn = boto.ec2.cloudwatch.connect_to_region('us-east-1',aws_access_key_id='*******************',
aws_secret_access_key='***************************',debug=2)

and

cn.get_metric_statistics(
    1800,
    datetime.datetime.utcnow() - datetime.timedelta(hours=24*12),
    datetime.datetime.utcnow(),
    'EstimatedCharges',
    'AWS/Billing',
    ['Sum'],
    dimensions={
       'Currency':'USD',
       'ServiceName': 'AmazoneEC2'
   }
)

But the data i got from this api doesn't match with the values shown in the aws console. What am i doing wrong here? Or if i want the billing reports of each day last month how should i modify the query?